1. Field of the Invention
The present invention relates to an apparatus for transporting substrates such as circular semiconductor wafers, rectangular glass plates, masks or the like, used in processes for manufacturing electronic circuit devices such as semiconductor devices or liquid crystal devices, and to a substrate holding apparatus which is required accompanying such transport.
2. Disclosure of the Related Art
Heretofore, of these types of substrate transport apparatus, with a transport mechanism which is mounted on a precision exposure apparatus used in a photolithography process (lithography involving painting-exposing-developing a photosensitive solution on a substrate), one substrate has been taken out from a carrier or an inline substrate stock section which stores several substrates (wafers or glass plates) to be subjected to exposure processing, and this has been automatically transported to a pre-alignment section by means of an articulated type robot arm or the like, to be further moved while being held at a desired pre-alignment accuracy on a vacuum attachment holder on a substrate stage in an exposure apparatus.
In the case of such a precision exposure apparatus, the vacuum attachment type substrate holder has the principal function of correcting the surface of the substrate to an extremely flat condition, so that the transfer image of the mask pattern to be exposed is formed on the exposure surface (reticle surface) of the substrate at a desired image quality (resolution, and degree of film reduction of the photosensitive layer etc.). Consequently, the mounting surface for the substrate holder is finished to an extremely flat surface of for example .+-.0.2 .mu.m. Recently, this type of substrate holder is made with a ceramic mother material, by depositing on the surface using a CVD method, a thin film exhibiting good electroconductivity (for example a layer of silicon carbide, titanium carbide or the like of a thickness of approximately 300 .mu.m) and then forming grooves for suction attachment in the thin film layer by mechanical cutting, or by optical polishing over the entire surface.
The reason for the high degree of flatness of the substrate holder is attributable to the decrease in the depth of focus originating in conjunction with the year by year increase in the resolving power of projection exposure apparatus, which constitute the main stream as precision exposure apparatus. That is, this is because when the depth of focus of the projection exposure apparatus becomes small, there is also the requirement for a high degree of flatness (local flatness) at the local region of the substrate surface corresponding to inside the projection field of the projection optical system.
Consequently, in particular with the projection type exposure apparatus, it is necessary to sufficiently maintain the accuracy of the flatness correction of the substrate by the substrate holder. Therefore it is desirable to obtain a uniform attachment force on the rear face of the substrate over the entire mounting surface of the substrate holder. However, here the-uniform attachment force does not necessarily mean that the entire substrate rear face provides vacuum attachment, but has the meaning that even in the case of vacuum attachment partially at a plurality of attachment grooves or the like, localized on the rear face of the substrate, if a desired degree of flatness is maintained considering the original shape and slight warping of the substrate, then with that, a uniform attachment force is obtained.
On the other hand, considered from the substrate transport mechanism side, if the construction is such that the transport arm contacts and holds the upper face of the substrate, then it is possible to place the substrate on the mounting face of the substrate holder, and remove a substrate which is attached to the mounting face. However here, in handling the substrate, since the upper face which is painted with the resist layer becomes the device forming face, then having the transport arm attached to and holding the upper face of the substrate becomes undesirable from the point of attachment of dust and dirt, or damage to the device forming surface.
Therefore, for the conventional substrate transport apparatus, there is known an apparatus as shown for example in Japanese Unexamined Patent Application, First Publication No. Hei 1-214042, which is provided with a vertically movable center-up portion (or three support members) located at a central portion of a substrate holder (or at three locations substantially equally displaced from the center of the holder) for lifting the substrate by a fixed amount from the holder mounting surface and supporting the substrate when the substrate is to be transported, and by inserting a transport arm into a space between the lifted substrate and the holder mounting surface, the transport arm can attach to and hold the rear face of the substrate.
Of the conventional wafer transport mechanisms disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 1-214042, in particular the transport mechanism provided with the vertically movable center-up portion at the central portion of the substrate holder, which adopts the system of supporting the rear face of the substrate on the upper end face of center-up portion by vacuum attachment and raises and lowers the substrate, is mounted onto a reduction projection type exposure apparatus NSR-1010G or NSR-1505G distributed by Nikon Company Limited in 1981 through 1983.
However, with the conventional apparatus, there is the problem that this can no longer necessarily and sufficiently cope with transporting and holding substrates which are becoming larger year by year. For example, in the production area of semiconductor circuit devices such as VLSI and ULSI, a production line for forming a pattern for a device chip on a semiconductor wafer of 300 mm diameter (previously 200 mm), has been introduced, with the requirement for safe and accurate transport of such large wafers, or stable retention thereof.
As indicated with the above conventional wafer transport mechanism, in the case where a cylindrical center-up section is only provided at the central portion of the substrate holder, then in order to stably and accurately hold a 300 mm diameter wafer at the central portion, the diameter of the upper end face of the center-up portion must be made greater than 30 mm (that is, greater than approximately 10% of the wafer diameter). Therefore, a large circular opening portion of more than 30 mm diameter must also formed in the central portion of the substrate holder for taking the center-up portion.
If in this way a large circular opening portion is formed in the central portion of the substrate holder, then on the rear face portion of a wafer mounted thereon, then naturally, there can be no holder upper end face (mounting face) for contact at the central portion, and hence flatness correction cannot be performed to sufficient accuracy, so that the problem of a deterioration in the local flatness arises.
On the other hand, in the case where three vertically movable support members are provided as illustrated for the conventional wafer transport mechanism, and apertures for respectively taking the three support members are formed at three locations in the substrate holder, if the positional spacing of the three support members is changed corresponding to the enlargement of the wafer, that is to say the positional spacing of the apertures at three locations on the substrate holder is changed, then it is considered possible to deal with the enlargement of the wafer without specially enlarging the diameters of the respective apertures.
However, even with three support members, if the diameters of the upper end portions of the respective support members remain as heretofore, the problem arises in accurate transport (delivery to the arm). This is because, if the diameter of the upper end face of the respective support members is small (for example around 4 mm), then the vacuum attachment force at the time of lifting and holding the wafer is insufficient so that at the time of wafer delivery between the transport arms, positional deviation of the wafer occurs (at worst around several hundred microns). This positional deviation is a serious problem, particularly in the loading of the unexposed wafer inside the exposure apparatus.
With the normal exposure apparatus, a fine alignment operation is performed by moving the wafer stage in the X-Y-direction with the wafer positioning accuracy set by a pre-alignment mechanism as a base, and positioning an alignment mark formed within a region of several 10s of microns on the wafer beneath an alignment microscope. Consequently, if during transport and delivery of the wafer onto the substrate holder of the wafer stage from the pre-alignment mechanism, a positional deviation occurs which excessively exceeds the positioning accuracy by the pre-alignment, then it is not possible to detect the alignment mark for the fine alignment operation.
In this case, with the exposure apparatus, due to error processing, subsequent operations are stopped, the exposure processing for that wafer is cancelled and the wafer returned to the standby position (for example the pre-alignment mechanism) and then forcefully transferred to the carry out position (for example the unload cassette). Therefore the processing throughput of the exposure apparatus drops.